In this work, an “off–on” type zinc ion fluorescent chemosensor H₂L with pyrene as a fluorophore was investigated. The chemosensor H₂L demonstrates a Zn²⁺-specific emission enhancement linearly with a 1 : 1 binding ratio even in the presence of a wide range of competing metal ions. This may be correlated with the efficient Zn²⁺ binding of H₂L, which promotes a good chelation-enhanced fluorescence (CHEF) effect and prevents the photoinduced electron transfer (PET) effect, and CN isomerization is inhibited. Importantly, the fabricated chemosensor can selectively distinguish Zn²⁺ from Cd²⁺. And the recognition behaviour has been proved experimentally and computationally.